I had a look at the cmake files in blender and noticed that the SUBDIRS() command is used instead of the newer ADD_SUBDIRECTORY() command.
The main difference between the two is that ADD_SUBDIRECTORY() is processed
immediately, while the directories added with SUBDIRS() are processed after
everything else in the CMakeLists.txt has been processed. This is usually not
what one would expect, so using ADD_SUBDIRECTORY() is usually better.
Example:

src/CMakeLists.txt:

subdirs(somedir)
set(FOO "foo")

src/somedir/CMakeLists.txt

message(STATUS "FOO: ${FOO}")


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With subdirs() the message() in the subdirectory will print "foo", even although FOO is set _after_ the subdirs() command.
When replacing the subdirs() in this example with add_subdirectory() FOO will be empty in somedir/CMakeLists.txt, as one would expect if everything is processed in order.


The attached changes all SUBDIRS() in blender to ADD_SUBDIRECTORY()
Please apply if you think it makes sense.

Alex